1. Field of the Invention
The present invention relates in general to a means for developing an electrostatic latent image and in particular to the means for developing the electrostatic latent image that enables to obtain a visible image with a high image density, an excellent sharpness and less fog.
2. Description of the Prior Art
In the past, there have been a powder cloud method to develop with a toner in the state of a powder cloud, an impression method wherein a uniform toner layer formed on a toner holder is contacted to a charge receptor or a microfield method, a magnetic brush developing method wherein a magnetic toner is used, and other developing methods for the electrostatic latent image wherein insulating one-component developer is used, in particular.
The drawbacks common to aforesaid developing methods of have been that the force to attract the toner to the toner holder is weak and therefore toner adheres also to the non-image area due to the physical adhering force, which tends to cause a fog. Further, these methods have been insufficient in image-sharpness and they have been problematic in practical use.
On the other hand, a jumping method has been used wherein the toner and the latent image are kept in the non-contact state and an alternating electric field is impressed for developing the image. This method has a technical difficulty because it is necessary to keep the thickness of a toner layer on the toner holder extremely thin.
Generally, the following factors are considered to be the cause of image quality deterioration in the developing method using one-component developer.
(1) Physical adherence of the toner to the non-image area
(2) Tailing phenomenon caused by the excessive development at the edge portion of a toner image
(3) Adherence of the toner with inverted polarity to the fringe portion of a toner image (fringe phenomenon)
In the developing method wherein a one-component developer is used, it is difficult to cause a sufficient electric field to be generated between a toner holder and an image holder as shown in FIG. 1(a) because no carrier is used and therefore an edge effect tends to be caused. In the figure, I represents an electrostatic latent image area, D represents a carrier and T represents a toner. (FIG. 1(b) shows a state of an electric field where a two-component developer is used.) Therefore, when a large area is to be developed with a high density, excessive toner adheres to the edge portion and the disturbance of the obtained image is caused when developing or transferring. This results in vignette characters and halftone dots.
On the other hand, fog is caused by adherence of toner due to a residual voltage of the photosensitive receptor as an image holder, or by Van der Waals force or a mirror-effect force. In the case of one-component toner, it is difficult to obtain a bias effect that can oppose these physical forces and consequently it is impossible to control the fog.
Further, in the case of a one-component toner, there inevitably exists toner charged with an inverted polarity and the adherence of toner to the fringe portion of an image (a fringe phenomenon) takes place, which leads to the phenomenon wherein resolution quality is lowered.